Moscow has strongly condemned the recent change of government in Ukraine, which came after months of street protests, more than 90 deaths and the flight of ousted President Viktor Yanukovych, a Russian ally.

Speaking during a visit to the Ukrainian capital Kiev on Tuesday, Mr Kerry said there was no indication at all that Russian citizens or Russian-speakers were in any danger in post-uprising Ukraine.

"It is clear that Russia has been working hard to create a pretext for being able to invade further," he said.

In one hint of progress, Ukraine's new Prime Minister Arseniy Yatsenyuk said consultations had taken place between Russian and Ukrainian ministers. He described them as "quite sluggish" but "first steps".

Crimea uncertainty

Moscow has tightened its grip over the Crimean peninsula after troops thought to be Russian or pro-Russian began taking control of strategic points on Saturday.

Key recent events

21 February - After months of protests, President Viktor Yanukovych signs a deal with opposition leaders to reduce his powers and hold early elections

22 February - Mr Yanukovych goes missing; protesters walk unopposed into official buildings as police abandon posts; parliament votes to impeach the president and calls elections for 25 May

23-26 February - Parliament names speaker Olexander Turchynov as interim president and Arseniy Yatsenyuk as PM; an arrest warrant is issued for Mr Yanukovych, who alleges a coup

Troops are surrounding Ukrainian military bases and other installations, while two Ukrainian warships are reported to be blocked by a Russian ship in the port of Sevastopol.

Ukrainian TV reported on Tuesday evening that armed men had attempted to take over an anti-aircraft missile base in Yevpatoria, on the coast north of Sevastopol.

Kiev and the West have alleged Russia is mounting an invasion of Crimea, which has a majority Russian-speaking population.

Mr Obama accused Russia of "seeking through force to exert influence on a neighbouring country".

"That is not how international law is supposed to operate," he said.

But at a lengthy news conference on Tuesday, in his first public comments on the issue, Mr Putin denied the heavily armed troops were Russian. He said they were "local self-defence forces" loyal to Moscow, protecting the bases from "nationalists" and "anti-Semites".

Russia, said Mr Putin, reserved the right to act to protect Russian citizens and speakers anywhere in Ukraine.

In Crimea and in eastern Ukraine, there have been shows of support for Russian intervention.

Pro-Russian troops have blocked Ukrainian soldiers in their barracks in Crimea

Russian President Vladimir Putin denies having deployed any troops in Crimea and says the soldiers are self-defence groups

But on Tuesday a peace rally in the eastern city of Donetsk urged Russia to stay away.

"We did not ask for help. I don't want him, Putin, to bring tanks here. I don't want them to shoot at my kids," one woman, Natalia Sytnik, told Reuters.

Mr Lavrov said the Russian position was "honest" and would not change, and that sanctions against Russia would be "counter-productive"

A court in Kiev quashed the decision by Crimea's parliament to sack the region's government and also ruled that the move to hold a local referendum on Crimea's status was illegal

Both the US and the EU have offered financial help to Ukraine, which is facing a growing economic crisis amid its severed ties with Moscow.

Mr Kerry took to Kiev a $1bn (£600m; 720m euros) package of energy subsidies, and told the crowds who remain in Independence Square after months of protests that President Obama "is planning more assistance".

The EU is considering paying the $2bn which Ukraine owes to Russia in gas bills, said EU Energy Commissioner Guenther Oettinger.